Reverse x-ray photoelectron holography device and its measuring method

ABSTRACT

[Problems] To provide a reverse X-ray photoelectron holography device, in which energy control and convergence are facilitated and a hologram of good contrast is obtained; and to provide its measuring method. 
     [Means for Solving Problems] A Reverse X-ray photoelectron holography measuring method where a measurement sample is irradiated with an electron beans, incident angle and rotation angle of the electron beam are varied by varying the posture of the measurement sample to the electron beam, and a variation in intensity of characteristic X-ray emitted when the measurement sample is excited is recorded as the atomic resolution hologram around the atom of a specific element, wherein, when the intensity is detected as the characteristic X-ray of the atom of the measurement sample, an object wave is generated as an electron wave scattered by the reference wave and the proximity atom in a holography where electrons incident to the measurement sample reach an atom generating specific X-ray as an electron wave, and an interference pattern is formed by compounding the reference wave with the object wave, thus monitoring the intensity of an electron beam.

TECHNICAL FIELD

The present invention relates to a inverse X-ray photoelectronholography measurement device for irradiating a measurement sample withan electron beam, detecting the intensity distribution pattern of anemitted characteristic X-ray, and using a Fourier transform to acquirean image, and also relates to a measurement method thereof.

BACKGROUND ART

X-ray photoelectron holography and fluorescent X-ray holographytechniques are effective in determining the crystalline structure ofminiaturized electronic devices or thin-film materials and otherindustrial materials, evaluating crystalline properties, and analyzinglocal structures and structural changes due to temperature variationsand chemical variations, and involve obtaining three-dimensional imagesof structures of the environment on the atomic level around an atom of aspecific element. These techniques are currently providing importantknowledge in explaining the functions of advanced materials, such asproviding analyses of dopants or surface adsorbates in semiconductors,or local structures in materials and are gaining attention from manyresearchers participating in materials development.

Next, the X-ray photoelectron holography technique will be described.This technique comprises acquiring and analyzing angular intensitydistribution patterns of photoelectrons emitted when a measurementsample is irradiated with monochromatic X-rays. Namely, atomic images ofthe measurement sample can be obtained by performing a Fourier transformon the resulting photoelectron distribution patterns. Because the atomicimages of the measurement sample can be observed in three dimensions,application [of these techniques] (*1) in a wide range of technicalfields is expected.

Examples of similar types of structural analysis techniques includefluorescent X-ray holography and laboratory fluorescent X-ray holographymeasurement devices designed for use in individual researchlaboratories. A laboratory fluorescent X-ray holography measurementdevice is described using FIG. 7. In FIG. 7, a powerfulrotating-anticathode X-ray generator (M21X, made by the now-defunct MacScience) having a maximum output of 21 kW was used as the X-raygenerator. A cylindrical graphite X-ray collecting element made byMatsushita Electric Industrial was used as a monochromator. An advantageof the cylindrical graphite X-ray collecting element is that the elementis formed to have a large curvature and superior precision. The opticalelement can receive white X-rays from a generator at wide solid angles,disperse or collect X-ray, and therefore obtain bright monochromaticX-rays. The monochromated X-ray energy can be adjusted by varying thedistance between the target and the graphite, and can be used to measurethe MoKα and Kβ characteristic X-rays in the case of a Mo target.

FIG. 8 is used to describe a high-speed fluorescent X-ray holographymeasurement device premised on the use of a radiation testing facilityat which highly intense monochromatic X-rays can be obtained.

An advantage of radiation X-ray in the high-speed fluorescent X-rayholography measurement device is that it is possible to obtain X-raysthat are brighter than from an ordinary X-ray generator, and operationbased on manipulating the energy variability produces a higher effect.Hologram measurements of radiation X-ray use numerous inverse modes inwhich a hologram can be measured by the interference between incidentX-rays.

On the other hand, because a provided beam can be used only for severaldays, measurement of a single hologram must be completed in severalhours. It is therefore important to develop a system which can detectfluorescent X-rays at a count rate of one million or greater counts persecond. One typical example of a high-speed X-ray detector is anavalanche photodiode (Avalanche Photo diode; APD), which can takemeasurements at up to 100 million counts per second. Because this methodof measurement does not have any energy resolution, a way to disperseonly fluorescent X-rays using a dispersive crystal must be devised. Inthis case, a crystal (LiF, graphite) processed into a cylindrical shapesuch as the one shown in FIG. 8( a) is used, the arrangement shown inFIG. 8( b) is adopted, and X-ray is dispersed and collected onto thedetection plane of the APD. When this system is used, a single hologramcan be measured in approximately two to three hours, and a sufficientnumber of hologram patterns can therefore be obtained even with beamtime of just several days.

It is known in the conventional analytical technical field that in orderto reproduce atomic images obtained by analyzing photoelectrons andfluorescent X-rays emitted from an atom, it is necessary to recordpatterns (holograms) with X-rays having a plurality of energies, and theuse of a large scale synchrotron radiation facility a plurality isindispensable for recording the patterns. Typically, using this type oftesting facility requires about half a year from when a test isrequested until when the test is carried out. Because many suchfacilities are public facilities, there are many cases in which they arenot readily available. Also, when using the facilities, the timeavailable for measurements is often limited and it is not possible tomeasure a large number of test samples. A state is thereby produced inwhich only a limited number of researchers can use [the facilities] (*1)under such limitations. Because of this situation, materials researchersand developers demanded development of a measurement device thatcontributes to expansion of the analytical technical field and basicresearch.

For example, in the prior art, the intensity of incident fluorescentX-rays was very weak and even if a hologram pattern could be obtained,the intensity of a fluorescent X-ray hologram being emitted from ameasurement sample varied by measurement sample in the measurementdevices provided at the laboratory level. Because about two months,which is more than approximately a few weeks, was required, there was alarge burden on researchers.

Because of this situation, a new design was devised for irradiation withX-rays, and it was proposed to use a laboratory fluorescent X-rayholography device in which resolution is improved by one to two ordersof magnitude compared with the planar crystals typically used in thefield by employing curved graphite as the dispersive crystal forincident X-rays to increase the intensity of incident X-rays. However,this device had the disadvantages that the wavelength could not befreely varied since a rotating-anticathode X-ray generator was used, andthat a sharp atomic image could not be obtained or the measurablesamples were limited since sufficiently intense X-rays could not beobtained. Even in cases in which a high-speed fluorescent X-raydetection system was incorporated into the above-mentioned technology,the intensity of detected fluorescent X-rays was several thousand countsper second, and it was confirmed that a measurement time of one month ormore was required.

Although many dopants of semiconductors or the like are nitrogen,phosphorus, and other X-ray elements (elements having an atomic numberlower than that of Ca), measuring the fluorescent X-rays of X-rayelements in the atmosphere is difficult due to air absorption, andholograms focusing on these atoms cannot be measured.

Collecting X-rays is difficult in cases in which the X-rays are used asan excitation source, and microcrystals or the like several microns insize cannot be used as measurement objects.

As stated above, since X-rays were used as an excitation source inconventional devices, energy control and convergence was difficult toaccomplish, and holograms having good contrast could not be obtained.Even when large-scale facilities were used, the devices were restrictedin a variety of ways and were not easy to use.

Patent Document 1: JP-A 2001-330571

Non-patent Document 1: Oyo Butsuri, Volume 72, Issue 7 (2003) pp.865-871, Issue Date Jul. 10, 2003.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention, which was devised to eliminate thedisadvantages of the above-mentioned prior art, is to provide a inverseX-ray photoelectron holography device that facilitates energy controland convergence and produces a highly contrastive hologram, and toprovide a measurement method thereof.

Means for Solving the Above-Mentioned Problems

In order to solve the aforementioned problems, the main point of thepresent invention is to use an electron beam as an excitation sourceinstead of X-rays in the inverse X-ray photoelectron holographyaccording to the present invention.

Means of solving the problems are as follows.

A inverse X-ray photoelectron holography device, characterized in havingmeans for irradiating a measurement sample with an electron beam,specifically, an electron beam controlled so as to ensure convergence toseveral electron-volts or less for the energy band, and to the order ofnanometers for the beam width; means for varying the incident angle andthe rotation angle of the electron beam by varying the orientation ofthe measurement sample; and means for recording the intensity variationsof a characteristic X-ray emitted when the measurement sample isexcited, the variation, being recorded as an atomic resolution hologramaround an atom of a specific element.

A inverse X-ray photoelectron holography measurement method includingirradiating a measurement sample with an electron beam, specifically, anelectron beam controlled so as to ensure convergence to severalelectron-volts or less for the energy band, and to the order ofnanometers for the beam width; varying the incident angle and therotation angle of the electron beam by varying the orientation of themeasurement sample in relation to the electron beam; and recording theintensity variations of a characteristic X-ray emitted when themeasurement sample is excited, the variations being recorded as anatomic resolution hologram around an atom of a specific element; theinverse X-ray photoelectron holography measurement method furthercomprising: detecting the intensity as a characteristic X-ray of theatom of the measurement sample, whereby an object wave is generated asan electron wave scattered by a reference wave and a neighboring atom inholography where electrons incident on the measurement sample reach anatom generating a specific X-ray as an electron wave; and monitoring theintensity of the electron beam for which an interference pattern isformed by compounding the reference wave and the object wave.

Further provided is a inverse X-ray photoelectron holography measurementmethod including irradiating a measurement sample with an electron beam,specifically, an electron beam controlled so as to ensure convergence toseveral electron-volts or less for the energy band, and to the order ofnanometers for the beam width; varying the incident angle and therotation angle of the electron beam by varying the orientation of themeasurement sample in relation to the electron beam; and recording theintensity variations of a characteristic X-ray emitted when themeasurement sample is excited, the variations being recorded as anatomic resolution hologram around an atom of a specific element, theinverse X-ray photoelectron holography measurement method comprising:using an inverse technique, which is an optical reciprocity theorem ofordinary X-ray photoelectron holography, to perform a Fourier transformon an electric signal of a hologram pattern recorded at variouswavelengths; and analyzing an atomic image of an atom peripherygenerated by a characteristic X-ray on the measurement sample.

EFFECT OF THE INVENTION

With the inverse X-ray photoelectron holography device according to thepresent invention, the accelerating voltage can be varied to measureholograms having a plurality of wavelengths, and highly precise atomicimages can be reproduced from a plurality of holograms having numerouswavelengths due to the use of an electron beam instead of X-rays.

The contrast of a fluorescent X-ray hologram obtained using X-rayinterference has an amplitude that is about 0.1% less than thebackground intensity. However, a hologram obtained by inverse X-rayphotoelectron holography, which measures electron interference patterns,has the advantage of extreme ease of measurement because the contrast isabout 1 to 10%.

A device which uses an electron beam has the further advantage of beingable to easily detect characteristic X-rays from a X-ray element beingreleased because the operation must always be conducted in a vacuum.

Microcrystals, materials, and other items in which the samples to bemeasured are several microns or less in size can be measured by applyingthe present invention to a commercially available scanning electronmicroscope or transmission electron microscope and ensuring convergenceof an electron beam to a size of several microns or less. The advantagesare that the ease of use is improved in comparison with conventionalmeasurement devices and that atomic images can be easily analyzed insidelaboratories without the use of large-scale facilities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a principle view showing inverse X-ray photoelectronholography according to the present invention;

FIG. 2 is a schematic view showing a inverse X-ray photoelectronholography measurement device according to the present invention;

FIG. 3 is a view showing a characteristic X-ray spectrum obtained by ainverse X-ray photoelectron holography measurement device;

FIG. 4 is a view showing oxygen K-line intensity variation obtained by ainverse X-ray photoelectron holography measurement device;

FIG. 5 is a conceptual view showing an electron image obtained byinverse X-ray photoelectron holography;

FIG. 6 is a principle view showing X-ray photoelectron holography;

FIG. 7 is a schematic view showing a laboratory fluorescent X-rayholography measurement device; and

FIG. 8 is a photograph and experiment arrangement diagram of ahigh-speed fluorescent X (*2) holography measurement device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detailhereinafter with reference to the annexed drawings.

FIG. 1 shows the principle of inverse X-ray photoelectron holography. Anembodiment of the present invention will be described using FIG. 1.Electrons (waves) are directed toward atoms that constitute a structurecharacteristic of the solid in a solid measurement sample from anelectron gun by using a inverse X-ray photoelectron holography device orthe like. The electrons are made to strike and excite the measurementsample, and characteristic X-rays are generated by the atoms thatconstitute the structure of the measurement sample. Because of thephenomenon in which the electrons arrive directly without beingscattered by other atoms when the characteristic X-rays are generated,the electron wave in holography formed in this manner is considered tobe a reference wave.

Next, a phenomenon occurs in which an electron wave that has caused asingle cycle of scattering on a neighboring atom plays the role of anobject wave. In this phenomenon, the reference wave and object waveinterfere with each other, forming an electron standing wave. Thestanding wave pattern is varied by the incidence angle of the electronsemitted from the electron gun, and the intencity of the electron wave atthe position of the characteristic X-ray emitting atom also varies.Namely, because the intencity of the electron wave is proportional tothe intencity of the characteristic X-rays generated by thecharacteristic X-ray generating atom, a hologram at the atomic levelaround the characteristic X-ray generating atom is recorded by varyingthe angle of the electron beam and measuring the intensity variation ofthe characteristic X-ray. This measurement method uses an opticalreciprocity theorem of X-ray photoelectron holography, a known methodwherein a monochromatic X-ray is incident on the atoms and the angulardistribution of the intensity of the emitted photoelectrons is measured.

A inverse X-ray photoelectron holography measurement device andmeasurement method that illustrates an embodiment of the presentinvention is described using FIG. 2.

A field emission scanning electron microscope (SEM) was used as theinverse X-ray photoelectron holography device shown in FIG. 2. By usingan electron source for emitting electrons, a turntable for holding orpositioning the measurement sample, and electrical control means (PC:personal computer, not shown) to control a motor (not shown, a typicalgeneral-purpose motor was used) in the device constitution, [the motor](*1) is rotated by the rotating shaft at a predetermined rotationalspeed. The incident angle can be varied by driving a motor that isseparate from the aforementioned motor (not shown). A measurement sampleis attached to the turntable, and an electron beam is directed towardthe measurement sample. Namely, the plane at which the electron beam isincident on the measurement sample is varied.

In order to obtain a sharp atomic image when implementing the presentinvention, a hologram must be recorded in a wide wave-number space. Toobtain this result, measurements are preferably performed in the widestpossible scan range so that the rotation angle φ ranges from 0 to 360degrees, and the incident angle θ ranges from 0 to about 80 degrees.

The electron beam emitted toward the measurement sample by thefield-emission scanning electron microscope is incident on themeasurement sample; the part of the measurement sample irradiated withthe electron beam, i.e., the atoms of the measurement sample, areexcited; and characteristic X-rays are emitted. A reference wave, whichis an electron wave that is not scattered, is generated; an object waveis then generated as an electron wave resulting from the scattering ofthe reference wave by a neighboring atom; and an interference pattern isformed by the reference wave and object wave. Because this interferencepattern has greater intensity than an interference pattern obtained byfluorescent X-ray holography, the measurement precision of the X-raydetector described hereinafter can be improved.

An X-ray detector for detecting the intensity of characteristic X-raysis then used. The characteristic X-rays which are incident on thedetector are replaced with an electric signal by the sensing of theX-ray detector. An SSD (semiconductor detector) was used as the X-raydetector in the present invention, but the detector itself may also bean SDD (silicon drift detector), a SiPIN diode, or another energydistribution X-ray detector, or a wavelength distribution detectorobtained by combining a dispersive crystal and a scintillation counteror the like.

The role of the detector is to detect interference (hologram) patterns.The electric signal output of the X-ray detector is transmitted via aninterface to an image processing device or PC (not shown) for storage.The PC has the role of performing a Fourier transform on the holograminformation (image), which is the electric signal obtained by the X-raydetector, to replace the signal with information in the form of anatomic image, and analyzing the image as necessary information.

The results obtained from an experiment performed based on the presentinvention are described hereinafter. Following is an example of ameasurement experiment involving inverse X-ray photoelectron holographyin which a field emission scanning electron microscope was prepared andSrTiO₃ (strontium titanate) monocrystals doped with 0.05 wt % Nb wasused for the sample. First, the accelerating voltage of the electronbeam (EB) emitted by the field emission scanning electron microscope wasset to 6 kV, and the beam was made to converge to a diameter of 100 mm.

A characteristic X-ray spectrum such as the one shown in FIG. 3, i.e., aspectrum of the measurement sample, was obtained as information obtainedwhen the measurement sample was irradiated with the electron beam. Forexample, the oxygen K-line was observed at channel 53, the strontiumL-lime was observed near channel 183, and the Ti K-line was observednear channel 453. The horizontal axis in FIG. 3 corresponds to themeasured X-ray energy, and the vertical axis corresponds to theintensity.

Next, the characteristic X-ray spectrum was measured within the angularscan ranges 45°. θ. 65° and 0°. φ. 100°. Particularly, the intensityvariation of the oxygen K-line was plotted in this case. FIG. 4 showsthe result. FIG. 4( a) shows the actual measurement values, and FIG. 4(b) shows the simulation. As can be confirmed by viewing FIG. 4, whencompared with a simulation based on the principle of X-ray photoelectronholography, the arc-shaped pattern shown by the dashed line can bereproduced, and nearly ideal measurement results are therefore obtained.

The angular scan range from measuring holograms was narrow in themeasurements according to the present invention, making it difficult toreproduce the resulting information by a Fourier transform, but becausethe measurements agreed with theoretical calculations, recordingholograms within a wide angular range made it possible to reproducehighly precise atomic images composed of constituent elements such asstrontium, oxygen, and titanium, as shown in FIG. 5.

A preferred inverse X-ray photoelectron holography device andmeasurement method have been described above, but the present inventionshall not be limited to this option alone and can be appropriatelymodified within a range that does not depart from the scope of thepresent invention.

FIG. 1 ELECTRON WAVE REFERENCE WAVE CHARACTERISTIC X-RAY ATOM GENERATINGCHARACTERISTIC X-RAY OBJECT WAVE SCATTERED ATOM FIG. 2 ELECTRON GUN LENSELECTRON BEAM X-RAY DETECTOR SAMPLE CHARACTERISTIC X-RAYS FIG. 6HOLOGRAM PHOTOELECTRON WAVE X-RAY FIG. 7 GONIOMETER CURVED GRAPHITESAMPLE ROTATING-ANTICATHODE X-RAY GENERATOR HIGH-SPEED SEMICONDUCTORDETECTOR FIG. 8 INCIDENT X-RAY SAMPLE FLUORESCENT X-RAY

CYLINDRICAL DISPERSIVE CRYSTAL

1. A inverse X-ray photoelectron holography device, characterized inhaving: an electron beam source for irradiating a measurement samplewith an electron beam; a control for varying the incident angle and therotation angle of the electron beam by varying the orientation of themeasurement sample in relation to the electron beam; and a detector forrecording the intensity variations of a characteristic X-ray emittedwhen the measurement sample is excited, the variations being recorded asan atomic resolution hologram around an atom of a specific element. 2.The inverse X-ray photoelectron holography device according to claim 1,wherein the electron beam is an electron beam controlled so as to ensureconvergence to several electron-volts or less for the energy band, andto the order of nanometers for the beam width.
 3. A inverse X-rayphotoelectron holography measurement method comprising: irradiating ameasurement sample with an electron beam; varying the incident angle andthe rotation angle of the electron beam by varying the orientation ofthe measurement sample in relation to the electron beam; and recordingthe intensity variations of a characteristic X-ray emitted when themeasurement sample is excited, the variations being recorded as anatomic resolution hologram around an atom of a specific element; theinverse X-ray photoelectron holography measurement method characterizedin comprising: detecting the intensity as a characteristic X-ray of theatom of the measurement sample, whereby an object wave is generated asan electron wave scattered by a reference wave and a neighboring atom inholography where electrons incident on the measurement sample reach anatom generating a specific X-ray as an electron wave; and monitoring theintensity of the electron beam for which an interference pattern isformed by compounding the reference wave and the object wave.
 4. Ainverse X-ray photoelectron holography measurement method, comprising:irradiating a measurement sample with an electron beam; varying theincident angle and the rotation angle of the electron beam by varyingthe orientation of the measurement sample in relation to the electronbeam; and recording the intensity variations of a characteristic X-rayemitted when the measurement sample is excited, the variations beingrecorded as an atomic resolution hologram around an atom of a specificelement; the inverse X-ray photoelectron holography measurement methodcharacterized in comprising: using an inverse technique, which is anoptical reciprocity theorem of ordinary X-ray photoelectron holography,to perform a Fourier transform on an electric signal of a hologrampattern recorded at various wavelengths; and analyzing an atomic imageof an atom periphery generated by a characteristic X-ray on themeasurement sample.
 5. The inverse X-ray photoelectron holographymeasurement method according to claim 3, wherein the electron beam is anelectron beam controlled so as to ensure convergence to severalelectron-volts or less for the energy band, and to the order ofnanometers for the beam width.
 6. The inverse X-ray photoelectronholography measurement method according to claim 4, wherein the electronbeam is an electron beam controlled so as to ensure convergence toseveral electron-volts or less for the energy band, and to the order ofnanometers for the beam width.
 7. The inverse X-ray photoelectronholography device according to claim 1, wherein the control includes aturntable for holding or positioning the measurement sample.